Effects of dissolved hydrogen on durability of silica glass under excimer laser irradiation.

As an evaluation of durability of synthetic silica glass under irradiation of KrF excimer laser (5.0 eV) or ArF excimer laser (6.4 eV), degradation of transmission was studied for a sample that contained 4×1018 molecules/cm3 H2 but was substantially free of other impurities. First, Khotimchenko’s method of H2 analysis was examined with specimens whose H2 contents were varied by vacuum degassing, covering three orders of magnitude down to H2-free (i.e., less than the instrumental detection limit of 5×1016). It was shown that this method was quite reliable.Then the effects of dissolved H2 were assessed. The hydrogen molecules were decomposed under excimer laser irradiation, and reacted with the E’ center. For KrF excimer laser irradiation, the degradation of transmission was found to be suppressed, when the concentration of hydrogen much increased. Whereas for ArF excimer laser irradiation, there seemed to exist an optimum concentration range.The author proposes a reaction model for the last observation as:≡ Si–O–Si ≡+ H2+hν→≡Si·+·O–Si≡+ H2→≡+Si–H + H–O–Si ≡, where ≡Si· is the E’ center, and ·O–Si≡ , the non-bxsridging oxygen hole center.